Inflatable bladders are sometimes used as mandrels to produce enclosed fiber reinforced polymer parts. Multiple plies of fiber reinforced polymer plies are laid up over the mandrel in order to form the plies into a desired part shape. The mandrel may be removed from the layup either after the layup is compacted and/or cured by deflating the bladder which allows the mandrel to collapse.
A problem sometimes exists in removing the collapsed mandrel which typically has flexible walls, because the mandrel may widen or expand along natural folding points. This widening effect may have the consequence of locking the mandrel within the inside walls of the compacted layup or the cured part. This mandrel lock-in may be particularly problematic in layups or parts having non-uniform cross sections presenting features such as joggles and/or bends that engage and interfere with withdraw of the widened mandrel. Mandrel lock-in may also be a problem in applications where the part has a uniform cross section but is relatively long, such as without limitation, an aircraft fuselage or a stringer, and may be due to the friction between the mandrel and the part.
Accordingly, there is a need for a collapsible mandrel that may not widen as it is being collapsed, thus allowing it to be more easily removed after a part has been laid-up and/or cured. There is also a need for a method of fabricating composite parts using such a mandrel.